This invention relates to the field of wicking and capillary action as are exemplified, for example, in the combination of liquid and solid elements in an electrical battery cell.
Certain electrical battery cells such as the sodium/sulfur cell employ structural elements which are not readily wetted by one or more of the battery constituent elements. Such elements are nevertheless often required to have a large area in intimate interface with the non-wetting battery constituent. In the sodium/sulfur battery cell, for example, it is common practice to use a solid-state ceramic electrolyte member for separating the sodium and the sodium reacting battery constituents. In this separation role, ions from the sodium constituent are transmitted through the ceramic electrolyte member in proportion to the wetted surface area interface between the sodium and ceramic materials. Since the electrical resistance and the current delivering capability of such a cell are directly related to this ion transmission capability, the achieved battery characteristics are often limited by ion transmission and the degree of wetting achieved by the cell constituents.
Heretofore, however, the achievement of satisfactory contact between a solid state electrolyte member and a liquid sodium reactant material has been accomplished with some difficulty and has often required the use of electrolyte surrounding structures made from graphite, for example, in order to provide a capillary space adjacent to the electrolyte member in which liquid sodium is communicated.
The difficulty in achieving satisfactory wetting between battery elements such as a ceramic electrolyte member and a liquid sodium constituent member, has also been described in the prior patent art. The patent of M. L. Wright et al. U.S. Pat. No 4,356,241, for example, describes this wetting difficulty and describes operating the battery cell at elevated temperatures, temperatures in excess of 400.degree. C., in order to achieve desirable wetting action. Other arrangements for increasing the wetting action in the sodium/sulfur cell in particular are described in the art recited in the Wright et al patent and in the specification and claims of the Wright et al patent itself. Generally, these arrangements include some form of chemical or physical treatment of the surfaces of the ceramic electrolyte member.
Other resolutions of the wetting difficulty in these cells have included the use of a wick member that is physically separate and distinct from the ceramic electrolyte member--in order to encourage better wetting action and travel of the liquid sodium constituent along the electrolyte member and away from the sodium reservoir.
In many battery arrangements, the provision of a relatively long and possibly wick action travel path for the liquid sodium is also desirable in order that the physical danger of locating sizable quantities of highly reactive materials such as sodium and sulfur in close physical proximity can be decreased. An example of separation arrangements for decreasing the possibility of accidental reaction from sodium/sulfur cell constituents, is to be found in the patent of T. Kogiso et al, U.S. Pat. No. 3,915,741.
The treatment of ceramic materials to achieve other desired conditions in a sodium/sulfur cell, such as low electrical resistance is described in patents represented by that of R. N. Singh, U.S. Pat. No. 4,381,216. The use of finely-divided materials in fabricating an electrical battery cell electrode member is also a known battery cell technique--as is illustrated, for example, in the patent of R. Bittihn et al. U.S. Pat. No. 4,439,502, and the patents of P. K. Church et al U.S. Pat. Nos. 4,207,391, 4,307,164, and 4,372,823.
None of these prior art treatments has, however, provided a fully satisfactory sodium to electrolyte contact arrangement for use in sodium/sulfur cells, and batteries of other nature.